COMMENTARY

Ancient Argentavis soars again

David E. Alexander* Department of Ecology and Evolutionary Biology, University of Kansas, 1200 Sunnyside Avenue, Lawrence, KS 66045

pproximately 6 million years feathers in its wing bones, and its wing (3), was still a huge , Ϸ33% heavier ago, in what is now Argentina, bones were long and robust, features not than a California .] This proce- an enormous bird ranged found in flightless (3). dure gave Argentavis a wingspan of 7 m across the region from the Chatterjee et al. (1) developed a pair and a wing area of 8 m2 (1). AAndes Mountains to the pampas. Imag- of computer models to analyze the Using their estimates of body mass ine a bird that has a condor-like body, flight of Argentavis. These models use a and dimensions as parameters in the weighs as much as a person, and has a stream-tube method, an approach devel- power model, Chatterjee et al. (1) gen- wingspan nearly that of a small airplane. oped to simplify analyses of helicopter erated a U-shaped power curve, typical Imagine further that this bird has a 55- performance (6). Using a method of flying and airplanes, with a cm-long skull with a massive, -like developed for helicopters makes sense minimum power requirement of Ϸ600 beak large enough to swallow a rabbit because, as various biomechanics re- W for sustained, level flapping flight. whole. Argentavis magnificens, the giant searchers have pointed out, flying Their estimate of the maximum sustain- teratorn, fits this description. In addition animals have more in common with he- able aerobic power available from Ar- to the general fascination stimulated by licopters than with fixed-wing airplanes gentavis’s muscles was only 170 W, any huge (but safely extinct) carnivore, (7, 8). Birds, in common with helicop- meaning that Argentavis was incapable the of this bird present paleontol- ters, use the same structures to produce of sustained flapping flight. How, then, ogists with a number of questions. Did it both lift and thrust (wings for birds and did it fly? fly? If so, was it a flapper like a goose rotors for helicopters), as opposed to Argentavis, like modern and or a soarer like its relatives, the con- separate wings (for lift) and engines (for vultures, would have been a soaring bird. dors? Some of the questions about the thrust) in conventional airplanes. Chat- Soaring birds maintain a shallow glide, flight in this huge bird have now been 3° or 4°, and take advantage of rising air answered by computer models described to stay aloft without flapping. The two in this issue of PNAS by Chatterjee et Birds, in common main sources of rising air available to land al. (1). Thanks to this work, we now birds are thermals and ridge lift. A ther- have a clearer picture of the flight abili- with helicopters, use mal occurs when uneven heating of ter- ties of this extinct creature. rain causes a mass of air to become Argentavis is a member of Teratorni- the same structures warmer than the surrounding air. This thidae, a family of large, extinct birds. mass of warm air rises, and if a thermal Ar- Although only partial skeletons of to produce both lift rises faster than the sinking speed of a gentavis have been found, they are very soaring bird in the warm air mass, the similar in general plan and proportion and thrust. bird is carried up by the thermal. Soaring to Teratornis merriami, a smaller tera- birds typically circle upward in thermals torn well known from Ͼ100 specimens and glide downward between thermals, collected at the Rancho La Brea tar pits terjee et al. developed two models, one sometimes covering enormous distances in in southern California (2). Earlier esti- to analyze the power requirements of the process (11). Ridge lift occurs when mates put the mass of Argentavis at Ϸ80 continuous, level, flapping flight, and wind blows up the slope of any inclined kg (3), but Chatterjee et al. (1) used a the other to simulate various flight ma- more sophisticated multivariate analysis neuvers of Argentavis. terrain, such as a hill or mountain. If the to arrive at an estimated body mass of To flesh out their model of flight vertical component of the wind speed is 70 kg. A good estimate of mass is criti- power, Chatterjee et al. (1) first needed greater than a soaring bird’s sinking cal because overall weight has a crucial to estimate the power available from the speed, then the bird can soar across the effect on flight characteristics like flight muscles of Argentavis, which they face of the slope, or slope soar, indefi- airspeed. based on the mass of the flight muscles. nitely without losing altitude. If the slope In living birds, the proportion of the is extensive, such as a long ridge or moun- Too Big to Fly? body mass devoted to flight muscles is tain chain, a soarer can also cover great Argentavis was so large, researchers have surprisingly constant over the whole distances by using ridge lift (8). long been intrigued about its flight ca- range of body sizes, with the main Challenging Takeoff pabilities. Large birds run up against downstroke muscles making up Ϸ15.5% a scaling problem because, as body size of the total body mass (9). Using this Perhaps the most intriguing result of increases, weight increases faster than muscle mass, Chatterjee et al. used data this work comes from the flight maneu- muscle power output, the former being on the metabolic rates and power out- ver simulation model of Chatterjee et al. a function of volume and the latter be- put of modern birds (10) to estimate the (1), which shows that both takeoff and ing related to cross-sectional area (4), so power Argentavis had available for landing would have been problematic very large birds have proportionately flight. The power analysis model also for Argentavis. Because its power output less powerful flight muscles (5). Flight incorporated wing dimensions, but no was less than one-third of the minimum requires a lot of power, and Argentavis is specimen of Argentavis has a complete so big that biologists have been puzzled wing skeleton, so they scaled up the di- about how it could have flown. The ana- mensions of the skeleton of T. merriami Author contributions: D.E.A. wrote the paper. tomical evidence that Argentavis flew is, to fill in the missing parts. [T. merriami, The author declares no conflict of interest. however, quite powerful. It had air-filled although only approximately one-fifth of See companion article on page 12398. bones, it had strong, appropriately the body mass and with wings only ap- *E-mail: [email protected]. spaced attachment points for secondary proximately half the span of Argentavis © 2007 by The National Academy of Sciences of the USA

www.pnas.org͞cgi͞doi͞10.1073͞pnas.0705515104 PNAS ͉ July 24, 2007 ͉ vol. 104 ͉ no. 30 ͉ 12233–12234 Downloaded by guest on October 2, 2021 needed for flapping flight, the model of Argentavis have indeed been found in find the necessary slope to permit a suggests that Argentavis could not have the Andean foothills. Others, however, takeoff. taken off from level ground in still air, have been found far to the southeast, on Some questions about flight in Argenta- even with a long takeoff run to gain the Argentine pampas. How could a vis remain unanswered. Could a burst of speed like a taxiing airplane. Chatterjee bird incapable of taking off from level a few seconds of anaerobic power have et al. describe two possible takeoff ground have made a successful living on given these huge birds enough power to methods. The first is to leap from a tall the pampas? take off without slopes or headwinds? perch, in which the bird would need to The modern pampas are a very windy Could they have landed on narrow ledges drop Ϸ20 m to build up enough speed place (3), and they were probably on cliffs or mountains, of the type favored to level out in still air or Ϸ12mina5 equally windy, although hotter and by condors? How common were the large m/s headwind. The other method is to drier, at the time of Argentavis (1). By thermals they would have required? And run down a slope, like a person launch- going beyond Argentavis, can this ap- ing a hang glider. Their model simulated proach tell us anything about flight in various conditions and found that Argen- This biomechanical other, very large, extinct flying animals? tavis could get airborne by running What about the other teratorns, for exam- down a 10° slope for Ϸ30 m in still air approach to studies ple, or the , extinct flying or 10 m with a 5 m/s headwind (1). relatives of the dinosaurs? As big as Argentavis The simulation model also suggests of extinct animals was, even larger flying animals once lived. Among pterosaurs, that landing on a level surface may have has given us detailed Pteranodon had a wingspan that was ap- been challenging for Argentavis. Its slow- proximately the same as Argentavis, al- est glide speed would have been Ϸ18 insight into how they though it probably weighed a bit less; m/s (below which it would stall and lose another , , had a its lift), far too fast to touchdown safely. may have lived. wingspan Ͼ50% greater than Argentavis, By using its wings for aerodynamic brak- making Quetzalcoatlus the largest known ing, the simulated bird was able to slow Ϸ flying creature. Can we learn anything to 6 m/s before touching down, which restricting their visits to the pampas to about power requirements and flight per- is still unsafe: A touchdown speed of 5 periods of predictably strong and nu- formance in pterosaurs using similar com- m/s is considered marginally safe at best merous thermals and strong winds puter models? If takeoff was very difficult for an this size (12). By gliding (probably summers), these huge birds for Argentavis, how much more difficult into a 5 m/s headwind and then braking, should have been able to fly over the would it have been for Quetzalcoatlus? the simulated Argentavis was able to pampas. Based on leg and pelvis geome- Would knowing Quetzalcoatlus’s flight bring its ground speed at touchdown try, Argentavis would have been a much power requirements shed any light on to Ͻ5 m/s. more agile walker than other large soar- whether it had an ectothermic or endo- ing birds like condors, and apparently thermic metabolism? This type of quanti- Pampas Paradox was capable of extensive walking, al- tative, biomechanical approach to studies With a body built for soaring, and the though it was not much of a runner (3). of extinct animals has given us surprisingly modern example of the This ability would have come in handy if detailed insight into how they may have demonstrating its feasibility, one might Argentavis ever found itself grounded on lived, and an approach similar to that of expect Argentavis to have spent its time level terrain. The big bird may have Chatterjee et al. (1) applied to other ex- slope soaring in the Andes. Some fossils been accustomed to lengthy hikes to tinct flyers should be equally enlightening.

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